I have the same problem. Defo made the mistake of leaving my greenhouse closed for a couple of days, when I went back it was so hot and humid in there.
The plants will be ok, just take the algae off. The soil might be quite dry underneath.
Defo need good ventilation, and not over water
:-)

So how does the water drain easily from the pots if they are sitting on a piece of wood? You need to get the pots into trays of some description and water from below not above and don't let them stand in water at all.